1. Field of the Invention
The present invention relates to a power semiconductor device for an igniter having an overheat protection function to protect a semiconductor switching device at an abnormally high temperature in an ignition system for an internal combustion engine.
2. Background Art
An ignition system for an internal combustion engine such as an automobile engine has, as components for generating a high voltage to be applied to an ignition plug, and a power semiconductor device incorporating an ignition coil (inductive load), a semiconductor switching device for driving the ignition coil and a circuit device (semiconductor integrated circuit) for controlling the semiconductor switching device. These components constitute a so-called igniter. The ignition system also has an engine control unit (ECU) including a computer. In ordinary cases, a protection function for protecting the semiconductor switching device in the event of occurrence of an abnormality in operation such as abnormal heat generation or application of an on-signal for a predetermined constant time period or longer by sensing the abnormality and forcibly shutting off the current flowing through the semiconductor switching device is provided in the power semiconductor device (see, for example, Japanese Patent Laid-Open No. 8-338350).
Because the overheat protection function is an operation according to self-protection of the power semiconductor device, timing of shutting off in the power semiconductor device is performed independently of ignition signal timing performed by the ECU. There is, therefore, a possibility of ignition occurring at an inappropriate time in the ignition sequence as a result of a shutoff operation in the overheat protection function to cause a backfire or knocking in the engine.
As a measure against the problem, methods have been proposed for softly shutting off the current so as not to cause ignition at the time of shutting off, i.e., for preventing an unnecessary ignition operation by setting the speed of shutting off the current flowing through the primary side of the ignition coil low enough to avoid inducing arc discharge on the ignition plug (see, for example, Japanese Patent Laid-Open Nos. 2001-248529 and 2008-45514).